1. Field of the Invention
The present invention relates to a multichannel SQUID flux meter adapted to detect magnetic fields from a number of regions at a time by the use of a plurality of SQUID flux sensors.
2. Description of the Background Art
High-sensitivity flux meters or magnetometers using superconducting quantum interference devices (SQUIDs) have been employed recently to measure small magnetic fields generated from a living body. By measuring the distribution of magnetic fields in the brain and heart, in particular, it becomes possible to presume current sources generating the magnetic fields. It is pointed out that this provides very significant information for diagnosis and is useful for identifying neural activity within a living organism. To measure the distribution of magnetic fields, a one-channel SQUID flux meter may be used to measure data from each region. With this method, however, it takes a long time to measure magnetic fields and thus a subject under examination will get tired. Moreover, current sources cannot be presumed with accuracy because magnetic fields in different locations cannot be measured simultaneously. For this reason there is a requirement for a multichannel SQUID flux meter which has an array of flux sensors and is adapted to measure magnetic fields from different regions simultaneously.
To meet this requirement, a multichannel SQUID flux meter system has been proposed in which there are provided processing circuits corresponding in number to the SQUID flux sensors, each of the processing circuits converting an output signal of a corresponding flux sensor to a magnetic-field signal (H. E. Hoenig et al., Biomagnetic multichannel system with integrated SQUIDs and first order gradiometers operating in a shielded room, Cryogenics, vol. 29, August, pp 809-813, 1989). In addition, a simplified system has been proposed in which each SQUID is driven by a different frequency and the outputs of the SQUIDs are multiplexed for transmission over a single line (Furukawa et al., Japanese Journal of Apply Physics, vol. 28, No. 3, March, 1989, pp L456-L458). Both of these systems use analog SQUIDs which provide a small analog signal.
As a digital SQUID, on the other hand, there is known a pulse-output-type SQUID in which a SQUID, consisting of a two-junction quantum interference device, is operated with an ac bias (Japanese Unexamined Patent Publication No. 63-290979). Also, there is known a system in which a voltage output of an analog SQUID, which operates with a dc bias, is applied to a superconducting comparator or 1-bit A/D converter to produce a pulse output (D. Drug, Cryogenics, vol. 26, pp 623-627, 1986). The digital SQUID has a feature that its output S/N ratio can be increased.
FIG. 1 illustrates a prior art multichannel SQUID magnetometer. In the figure, 1 and 2 denote room-temperature sections placed in a room-temperature environment, while 3 denotes a low-temperature section which is maintained at an extremely low temperature. The low-temperature section 3 has SQUIDs 4, 5, and 6 for measuring magnetic flux density; AND gates 7, 8, and 9 adapted to select a measurement channel; feedback circuits 10, 11, and 12 for applying feedback fields to the SQUIDs 4, 5, and 6; a superconducting decoder 13 for enabling the AND gates 7, 8 and 9 selectively; and a control cable 14 for supplying parallel channel select signals to the decoder from outside. The room- temperature section 1 has amplitude regulators 15, 16, and 17 for regulating the magnitude of high-frequency bias currents flowing through the SQUIDs (each including a coil which couples with a magnetic sensor and a magnetic feedback coil) and current sources 18, 19, and 20 for supplying high-frequency bias currents to the respective individual SQUIDs.
The operation of the prior art SQUID magnetometer is as follows.
In a channel selected by an enabled one of the AND gates 7, 8, and 9, an output current corresponding to the number of output pulses of the corresponding SQUID flows through the feedback coil placed in the neighborhood of the SQUID, so that a feedback magnetic field is generated in the direction to cancel out an external magnetic field applied to the SQUID.
Radio-frequency pulses are output according to variations of the external field until the external field has been canceled out by the feedback field. By counting the number of high-frequency pulses by means of a processor 21, it becomes possible to measure the density of magnetic flux externally applied to each of the SQUIDs 4, 5, and 6.
FIG. 2 illustrates a relationship between gate select pulses and SQUID output pulses.
In FIG. 2, (a), (c), and (e) indicate SQUID output pulses when channels CH1, CH2, and CHn are selected, while (b), (d), and (f) indicate AND-gate enabling pulses for selecting channels CH1, CH2, and CHn, respectively.
As can be seen, in the prior art, the output pulses of the SQUID in each channel correspond in number to the magnetic flux density applied thereto.
The output pulses of the channels selected on a time-division basis are fed into the processing unit 21 in the room temperature section 2, so that they are counted by its counter to compute magnetic flux density.
With the conventional SQUID magnetometer described above, there is a limit to speeding up of measurement of magnetic flux density because high-speed pulses output from each SQUID are fed into a counter maintained at room temperature. Moreover, the conventional SQUID magnetometer is not sufficient to accommodate an increased number of channels.
Furthermore, with the conventional multichannel SQUID magnetometer, since each SQUID is adapted to operate only when its associated channel is selected by the decoder, measurement has to be initiated from a start level each time a channel is selected, requiring a certain period of time until the measured value has been settled. Therefore, simultaneous measurement of a large number of channels is difficult.
This will be explained more specifically. From a consideration of conventional examples , the frequency bandwidth required for biomagnetism measurement is several hundreds of hertz . Assuming the frequency bandwidth to be 300 Hz , all the channels will have to be scanned every 1/600 Hz, i.e. , 1.7 ms as can be seen from the sampling theorem . Assuming now that the number of channels is 100, then a time interval allotted to each channel will be 1.7 ms/100=17 .mu.s. When a digital SQUID is operated in a feedback mode so as to obtain a magnetic flux signal, on the other hand, the magnetometer has the same type of transfer function as a first-order lowpass filter for a small signal such as biomagnetism (see Fujimaki et al. , J. Apply. Phys., 65(4), pp 1626-1630, 1989) and its cutoff frequency is given by EQU .omega.o=2 fB.multidot..DELTA..PHI..multidot.dP/d.PHI.
where fB is a bias frequency applied to the SQUID, .DELTA..PHI. is a quantity of feedback per pulse, P is the switching probability and .PHI. is the signal magnetic flux.
Assuming that the bias frequency=1 MHz, .DELTA..PHI.=1.times.10.sup.-5 .PHI.o, dp/d .PHI.=100/.PHI.o, fo would be about 320 Hz. However, in order to catch up with input magnetic flux during the time interval allotted to each channel, i.e., 17 .mu.s, the frequency bandwidth is required to be more than 20 KHz (the bandwidth required to catch up with more than 90% of the input flux). Thus, the bias frequency must be further increased by at least two orders of magnitude. To increase the sensitivity of the magnetometer, .DELTA..PHI. must be decreased. If .DELTA..PHI. were decreased, the bias frequency would have to be further increased to compensate for a decrease in .DELTA..PHI..
By the way, a matter of importance in the multichannel version of the SQUID magnetometer is how to decrease the number of signal lines to the SQUIDs. The reason is that SQUID sensors are usually placed in a low-temperature environment cooled by a coolant such as liquid helium. Also, if the number of channels were increased, the signal lines connecting the SQUID sensors to circuitry kept at superconducting or low temperature would be increased and hence the consumption of the expensive coolant would also be increased. With the conventional analog SQUID system, a processing circuit (maintained at room temperature) is provided for each of the SQUID sensors. Thus, output lines and feedback lines corresponding in number to channels are required. In the frequency multiplexing, although only one output line suffices, as many feedback lines as there are channels are required.
As described above, in order to realize a multichannel SQUID magnetometer with more than 100 channels, a bias frequency as high as several hundreds of MHz is required. In the case of multichannel, a problem which arises with the operation in such a high frequency range is that crosstalk is liable to occur between channels. Moreover, the conventional multichannel SQUID magnetometer suffers from a problem that an increase in the number of channels results in an increase in the consumption of a coolant such as liquid helium.